Band-rejection filter using parallel-connected commutating capacitor units

ABSTRACT

Plural commutating capacitor, 2-terminal, impedance units are included in parallel-connected branch circuits which are, in turn, connected in series in a signal path to suppress a predetermined signal frequency as well as certain harmonic frequency-rejection bands that would otherwise be present if a single commutating capacitor unit were employed. The plural units are essentially the same in structure and are driven for commutation switching at the same frequency but in different phases with respect to one another.

United States at'ent n 1 Bahler et a1.

1451 Sept. 11,1973

[22] Filed:

1 BAND-REJECTION FILTER USING PARALLEL-CONNECTED COMMUTATING CAPACITORUNITS [75] Inventors: Lawrence George Bahler, Berkely Heights; JosephHenry Condon, Summit, both of NJ.

[73] Assignee: Bell Telephone Laboratories,

Incorporated, Berkeley Heights, NJ.

July 24, 1972 [21] Appl. No.: 274,488

[52] US. Cl. 333/70 A, 328/167, 333/76 [51] Int. Cl. H0311 7/10, H03h7/16 [58] Field of Search 333/70 R, 70 A, 76;

[56] References Cited UNITED STATES PATENTS 2,752,491 6/1956 Ringoen333/70 A 3,403,345 9/1968 Frank et a1. 328/165 3,514,726 5/1970Poschenrieder... 333/70 R 3,526,858 9/1970 Heinlein et a1 333/70 RPrimary Examiner-Rudolph V. Rolinec Assistant Examiner-Marvin NussbaumAtt0meyW. L. Keefauver [57] ABSTRACT Plural commutating capacitor,Z-terminal, impedance units are included in parallel-connected branchcircuits which are, in turn, connected in series in a signal path tosuppress a predetermined signal frequency as well as certain harmonicfrequency-rejection bands that would otherwise be present if a singlecommutating capacitor unit were employed. The plural units areessentially the same in structure and are driven for commutationswitching at the same frequency but in different phases with respect toone another.

13 Claims, 4 Drawing Figures 1 ccu wv PAIENTED 381 Hm CCU I I I CCU .FIG. 3

DATA {L SHIFT REG.

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I BAND-REJECTION FILTER USING PARALLEL-CONNECTED COMMUTATING CAPACITORUNITS BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to dynamic band-rejection filters utilizingcommutating capacitor units.

2. Description of the Prior Art It is known in the art to employ acommutating capacitor unit as a two-terminal impedance device insubstitution for a parallel inductance capacitance (L-C) circuit. Theconstruction, operation, and application of such units are set forth inthe copending J. H. Condon U.S. Pat. application Ser. No. 254,384, filedMay I8, 1972, which is entitled Commutating Capacitor Impedance Device,"and which is assigned to the same assignee as the present application.Although commutating capacitor units resemble corresponding L-C circuitsin certain respects, there are also certain differences. One suchdifference involves the fact that commutating capacitor units, whichdisplay a resonant circuit response at the recurrence frequency ofcapacitor switching commutation, also display similar responses atcertain harmonics of that frequency. For example, a commutatingcapacitor unit, which is connected in series in a signal path, effectssuppression of signals at the commutation frequency f of the unit.However, the unit also displays signal suppression and harmonicgeneration effects at certain harmonics, notably the th, 7th, 11th,13th, et cetera. These are hereinafter designated simply harmoniceffects. In certain electric signal systems, these harmonic effects mayappear in or very close to a desired signal passband. Of course, when aunit is working as described in a band-rejection mode, it is difficultto re-establish a desired response characteristic at theharmonic-rejection bands. However, if the unit is being utilized in abandpass mode, i.e., in which the unit is connected in shunt across thesignal path, the closest harmonic which is passed is the 5th; and it canoften be excluded by a comparatively simple low-pass filter.

STATEMENT OF THE INVENTION The foregoing problem of harmonic effects inthe output of a band-rejection filter is resolved in an illustrativeembodiment of the present invention by connecting plural, substantiallyidentical commutating capacitor units in parallel with one another andin series in a signal path. These units are driven for commutationswitching in different phases of the same commutation recurrencefrequency. The number of parallelconnected units determines the harmonicrejection band effects and harmonic generation that are suppressed. Itis to be noted here that the utilization of the terms rejection andsuppression" do not generally indicate total elimination of certainsignal components, but they indicate instead a substantial reduction inthe response with respect to those components as compared to othersignal components which may also appear in the signal path.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of theinvention and various features, objects, and advantages thereof may beobtained from a consideration of the following detailed description inconnection with the appended claims and the attached drawings in which:

FIGS. 1A and 1B illustrate a schematic diagram of a commutatingcapacitor unit and a schematic representation of such a unit as employedelsewhere in the drawing;

FIG. 2 is a schematic diagram of a band-rejection filter in accordancewith the present invention; and

FIG. 3 is a logic diagram of circuits for generating commutating drivesignals in different phases for utilization in the filter of FIG. 2.

DETAILED DESCRIPTION In FIG. 1A there is presented in simplified formone embodiment of a commutating capacitor unit of the type described inthe aforementioned Condon application. Briefly, three capacitors 10, 11,and 12 are connected in a delta circuit configuration having apexterminals l3, l4, and 15, which are connected in different paircombinations between an input connection 18 and an output connection 19of the unit. The different combinations of connections are achieved by acommutating switching arrangement which, in effect, rotates the deltacircuit clockwise at a frequency f,,, so that the apex terminals of thedelta circuit are alternately brought into contact with different setsof three contacts of a commutating switch which includes contacts 20,21, 22, 23, 24, and 25. In actual practice the commutating switching isadvantageously accomplished by electronic switching arrangements, two ofwhich are disclosed in the aforementioned Condon application. All of thethree capacitors of the delta circuit advantageously have approximatelythe same capacitance C. Signal conditions observed across the device ofFIG. 1A, when an electrical signal is applied across the connections 18and 19, resemble the response of a parallel inductance-capacitancecircuit. In particular, maximum response is realized for an input signalfrequency which is equal to the commutation frequency f,,.

FIG. 1B is a schematic representation of the twoterminal impedancedevice illustrated in FIG. IA. This representation is normallyconsidered to include the means, of whatever form, utilized forachieving the commutation switching. However, circuits are separatelyshown herein for producing drive signals for actuating the switchingmeans.

FIG. 2 illustrates an unbalanced band-rejection filter circuit utilizingthe present invention. Three commutating capacitor units 28, 29, and 30are connected in series in each of three parallel-connected branchcircuits 31, 32, and 33 between terminals 36 and 37 in the throughsignal path of the filter. One ofthe filter input terminals 38 isconnected directly to the terminal 36 and the other is a ground circuitwhich extends through the filter to one of the output terminals 39thereof. Three current limiting resistors 40, 41, and 42 ofapproximately equal value are connected in series with the commutatingcapacitor units in the branch circuits 31, 32, and 33, respectively;and, along with the capacitors of those units, they influence thebandwidth of the filter.

An operational amplifier 43 is also connected in series in the throughsignal path of the filter between terminals 37 and 39; and itsinverting, or negative, input connection is directly coupled to theterminal 37. The noninverting input connection of amplifier 43 isconnected to ground. A gain determining feedback resistor 46 is providedaround the amplifier 43 so that the negative input of the amplifierprovides a convenient current summing node for currents from the branchcircuits 31, 32, and 33. A terminating resistor 47 may be connectedbetween the output terminals 39.

Although three commutating capacitor units are shown in theband-rejection filter of FIG. 2, either fewer units or more units may beadvantageously employed depending upon the filter output responserequirements, as will be hereinafter described. All of the units havesubstantially the same electrical configuration and utilize capacitorsof the same capacitance. It has been found that plural commutatingcapacitor units connected as shown and driven at the same phase andfrequency, assuming the units to be substantially identical instructure, actually produce a signal response effect which is similar tothat of a single commutating capacitor unit. However, when the units aredriven at the same frequency and at appropriately selected differentphases with respect to one another, different ones of the aforementionedharmonic effects are suppressed. Thus, the three units in FIG. 2 aresupplied with commutation switching drive signals at frequencies 6f 6]and 6f,,, respectively, for a filter which is to have the frequency f atthe center of its fundamental band rejection response. The drive signal6f is advantageously delayed in phase by electrical degrees as measuredon the filter input signal wave component at frequency f which is to besuppressed. In like manner for the case in which three commutatingcapacitor units are employed, the unit is driven by a signal 6]" whichis delayed by 40 electrical degrees with respect to the wave 6f and 20degrees with respect to the wave 6j' In more general terms, it has beenfound that the phase differences between the drives for the differentcommutating capacitor units should be 60 degrees/N where N is equal tothe number of parallel-connected commutating capacitor unit branchsignal paths employed'in the filter. Thus, for the case just outlinedwith respect to FIG. 2, N equals 3; and there is a 20-degree phasedifference between successive ones of the drive signals for units 28,29, and 30.

FIG. 3 illustrates schematically one arrangement for advantageouslyobtaining the commutating drive signals utilized in FIG. 2. Thus, amaster clock source 48 is operated at a frequency 6Nf which correspondsto l8f for the example used herein, and its output is coupled through adivide-by-N, where N equals 3 for the example, circuit 49 to produce the6f,, drive signal. The output of circuit 49 is also applied to the datainput of an (N-l) shift register 50 which receives shift pulses directlyfrom the source 48. Conventional positive-edgetriggered logic isadvantagously employed for circuit 49 and register 50. Thus, by the timethe output of circuit 49 changes state in a positive-going sense inresponse to a positive-going pulse from source 48, the clockingcapability of that pulse has passed and register 50 cannot be operateduntil the next succeeding pulse from source 48. Corresponding outputs oncircuits 51 and 52 from the first and second stages, respectively, ofregister 50 provide the 6]} and 6f signals which differ in phase by 20degrees and degrees, respectively, with respect to the 6f,, signal andby 20 degrees with respect to one another. The aforementioned phasedifference measures are, of course, indicated with respect to the f,,input signal at the center of the band-rejection characteristic of thefilter in FIG. 2.

It has been previously indicated that a commutating capacitor unitconnected in a band-rejection filter arrangement suppresses thefundamental frequency of commutation, as well as all odd harmonicsthereof which are not multiples ofthree. The utilization ofparallel-connected, commutating capacitor unit, branch circuits, inseries in the signal path of a bandrejection filter, causes furthersuppression of certain ones of the aforementioned odd harmonic responseand generation effects. In general terms, it can be said that allharmonic effects are suppressed except those which are indicated by theexpression (6*N*Iil wherein N is equal to the number of commutatingcapacitor unit branch circuits, and I is any positive integer.

Thus, if a single commutating capacitor unit is employed in the circuitof FIG. 2, N equals 1 and the harmonic effects which are not suppressedare the 5th, 7th, llth, 13th, 17th, 19th, et cetera, as I increases.Similarly, if N equals 2 the unsuppressed harmonic effects are the llth,13th, 23rd, 25th, 35th, et cetera; and if N equals 3, the unsuppressedharmonic effects are the 17th, 19th, 35th, 37th, et cetera. Thus, itwill be seen that as commutating capacitor unit branch circuits areadded between terminals 36 and 37 in FIG. 2, the frequency of the lowestharmonic effect which is not suppressed becomes higher; and the spreadbetween adjacent pairs of unsuppressed harmonic effects increases.

The harmonic suppression effect realized in the band-rejection filter ofFIG. 2 is understood to occur as a function of the sum of the responsesof the parallelconnected branch circuits such as the circuits 31 through33. Thus, similar harmonic suppression effects can be expected in othercircuit applications wherein currents from parallel-connectedcommutating capacitor unit branch circuits can be summed.

Although the present invention has been described in connection with aparticular embodiment, it is to be understood that additionalembodiments, applications, and modifications which will be obvious tothose skilled in the art are included within the spirit and scope of theinvention.

What is claimed is:

1. In combination,

a plurality N of commutating capacitor units,

means for connecting said units in parallel signal paths, and 7 meansfor driving said units in different phases with respect to one anotherfor commutating capacitor connections in such units.

2. The combination in accordance with claim I in which each of saidunits comprises,

an input connection and an output connection,

a plurality of capacitors interconnected with one another in a circuithaving a predetermined number, greater than two, of terminals, and

means for coupling the terminals of different paired combinations ofsaid terminals to said input and output connections, respectively, in apredetermined recurring sequence of time intervals, each of saidcombinations including in series therebetween at least two of saidcapacitors.

3. A band-rejection filter comprising,

a plurality N of commutating capacitor units, each of said unitsincluding a plurality of capacitors connectible in recurrent alternatingconnection cycles,

means for connecting said units in parallel-connected branch paths of asignal path in said filter, and

means for driving said units in different phases with repsect to oneanother for commutating capacitor connections in said units through saidrecurrent cycles.

4. The filter in accordance with claim 3 in which said driving meanscomprises,

means for driving said units in different phases of the same frequencywhich are 60 degrees/N apart from the phase of every other one of saidunits.

5. The filter in accordance with claim 3 in which said driving meanscomprises,

means for fixing said different phases with respect to one another forsuppressing all harmonic effects of said filter except those atharmonics satisfying the expression (6*N*El) wherein l is any positiveinteger.

6. The filter in accordance with claim 3 in which,

two of said units are provided, and

said driving means drives said units at the same commutating frequencywith thirty degrees difference in phase as measured on the waveform of asignal at the frequency at the center of the fundamental frequencyrejection band of said filter.

7. The filter in accordance with claim 3 in which,

three of said units are provided, and

said driving means drives said units at the same commutating frequencywith twenty degrees difference in phase as measured on the waveform of asignal at the frequency at the center of the fundamental frequencyrejection band of said filter.

8. The filter in accordance with claim 3 in which,

said connecting means comprises a plurality N of current limitingresistors, and

means for connecting each of said resistors in series with an output ofa different one of said units in its respective parallel-connectedbranch path.

9. The filter in accordance with claim 3 in which,

input and output connections are provided, and

said connecting means includes,

an operational amplifier, and means for connecting saidparallel-connected units and said amplifier in series between said inputand output connections.

10. The filter in accordance with claim 9 in which said amplifiercomprises an inverting input connection which is connected in saidseries signal path to a common terminal of said parallel-connected unitswhich is remote from said input connections,

said amplifier also includes a noninverting input connection which iscoupled to a reference potential, and

a feedback resistor is connected between an output of said amplifier andsaid inverting input connection.

11. The filter in accordance with claim 10 in which,

said output connections include a terminating resistor connectedtherebetween, and

means for connecting one of said output connections to said referencepotential terminal.

12. The filter in accordance with claim 3 in which said signal pathwherein said parallel-connected commutating capacitor units areconnected is a series signal path connecting an input terminal of saidfilter with its corresponding output terminal.

13. The combination in accordance with claim 1 in which each of saidunits comprises,

a plurality of capacitors interconnected with one another, and

means for commutating said interconnected capacitors through apredetermined sequence of connection combinations between input andoutput connections of said unit, each of said combinations including aplurality of capacitors connected between said input and outputconnections and each such capacitor being included in at least two ofsaid combinations in said sequence.

i II! l

1. In combination, a plurality N of commutating capacitor units, meansfor connecting said units in parallel signal paths, and means fordriving said units in different phases with respect to one another forcommutating capacitor connections in such units.
 2. The combination inaccordance with claim 1 in which each of said units comprises, an inputconnection and an output connection, a plurality of capacitorsinterconnected with one another in a circuit having a predeterminednumber, greater than two, of terminals, and means for coupling theterminals of different paired combinations of said terminals to saidinput and output connections, respectively, in a predetermined recurringsequence of time intervals, each of said combinations including inseries therebetween at least two of said capacitors.
 3. A band-rejectionfilter comprising, a plurality N of commutating capacitor units, each ofsaid units including a plurality of capacitors connectible in recurrentalternating connection cycles, means for connecting said units inparallel-connected branch paths of a signal path in said filter, andmeans for driving said units in different phases with repsect to oneanother for commutating capacitor connections in said units through saidrecurrent cycles.
 4. The filter in accordance with claim 3 in which saiddriving means comprises, means for driving said units in differentphases of the same frequency which are 60 degrees/N apart from the phaseof every other one of said units.
 5. The filter in accordance with claim3 in which said driving means comprises, means for fixing said differentphases with respect to one another for suppressing all harmonic effectsof said filter except those at harmonics satisfying the expression(6*N*I + or - 1) wherein I is any positive integer.
 6. The filter inaccordance with claim 3 in which, two of said units are provided, andsaid driving means drives said units at the same commuTating frequencywith thirty degrees difference in phase as measured on the waveform of asignal at the frequency at the center of the fundamental frequencyrejection band of said filter.
 7. The filter in accordance with claim 3in which, three of said units are provided, and said driving meansdrives said units at the same commutating frequency with twenty degreesdifference in phase as measured on the waveform of a signal at thefrequency at the center of the fundamental frequency rejection band ofsaid filter.
 8. The filter in accordance with claim 3 in which, saidconnecting means comprises a plurality N of current limiting resistors,and means for connecting each of said resistors in series with an outputof a different one of said units in its respective parallel-connectedbranch path.
 9. The filter in accordance with claim 3 in which, inputand output connections are provided, and said connecting means includes,an operational amplifier, and means for connecting saidparallel-connected units and said amplifier in series between said inputand output connections.
 10. The filter in accordance with claim 9 inwhich said amplifier comprises an inverting input connection which isconnected in said series signal path to a common terminal of saidparallel-connected units which is remote from said input connections,said amplifier also includes a noninverting input connection which iscoupled to a reference potential, and a feedback resistor is connectedbetween an output of said amplifier and said inverting input connection.11. The filter in accordance with claim 10 in which, said outputconnections include a terminating resistor connected therebetween, andmeans for connecting one of said output connections to said referencepotential terminal.
 12. The filter in accordance with claim 3 in whichsaid signal path wherein said parallel-connected commutating capacitorunits are connected is a series signal path connecting an input terminalof said filter with its corresponding output terminal.
 13. Thecombination in accordance with claim 1 in which each of said unitscomprises, a plurality of capacitors interconnected with one another,and means for commutating said interconnected capacitors through apredetermined sequence of connection combinations between input andoutput connections of said unit, each of said combinations including aplurality of capacitors connected between said input and outputconnections and each such capacitor being included in at least two ofsaid combinations in said sequence.